To understand the functions of the different nonmuscle myosin II isoforms (NM IIs) during mouse heart development we crossed mice expressing cre-recombinase controlled by various promoters with floxed NMHC II-A and II-B mice. Nkx2.5-Cre mediated ablation of NM II-B (BNkx/BNkx) in cardiomyocytes caused cytokinesis defects similar to those seen in NM II-B ablated mouse hearts. ANkx/ANkx cardiomycytes on the other hand showed no defects in heart development. Mice ablated for NM II-B by SM22 alpha-Cre in epicardial and myocardial cells (BSM22/BSM22) survived to adulthood. BSM22/BSM22 hearts showed marked abnormalities in coronary vessel remodeling. These mice developed arrhythmogenic right ventricular cardiomyopathy (ARVC) and died suddenly at the age of 6 months. BSM22/BSM22 mice resemble humans with ARVC with the exception that they lack fat cell deposition in the myocardium. Defects in coronary vessels were confirmed in mice ablated for NM II-B specifically in epicardial cells by crossing to WT-1-Cre mice however BWT/BWT mice died during embryonic development for unknown reasons. Consistent with abnormal coronary vessel formation, epicardial explants showed that genetic ablation of NM II-B or inhibition of all NM II activity by blebbistatin impaired epicardial EMT. Surprisingly, despite significant expression of NM II-A in epicardial cells, ASM22/ASM22 mice showed no obvious heart defects and survived to adulthood. We next ablated NM II in endocardial cells using Tie2-Cre mice. Except for a minor delay in coronary plexus coverage in embryonic hearts, BTie2/BTie2 mice survived to adulthood with no obvious defects. In contrast ATie2/ATie2 mice showed defects in coronary vessel formation in embryonic hearts and died during embryonic development. Of note ATie2/ATie2 embryonic mouse hearts at E14.5 showed marked impairment in coronary coverage over the heart surface indicting a sprouting defect during coronary vessel formation. Furthermore compound Tie2-Cre mice ablated for one allele of NM II-A and both alleles of NM II-B (A+/ATie2; BTie2/BTie2) developed a severe defect in coronary coverage suggesting that NM II-B functions when NM II-A expression is limited. These results reveal that NM II-A and II-B function in two different developmental processes during coronary vessel formation in the mouse heart. NM II-A regulates coronary plexus expansion over the heart surface through the endocardial/endothelial cell lineage. NM II-B regulates coronary vessel remodeling through the epicardial derived cell lineage. We are also studying mouse models of human diseases. The three isoforms of nonmuscle myosin II (NM II) play a variety of roles during mouse embryonic development, and mutations in two of the genes encoding the heavy chains (MYH9 which encodes NM II-A and MYH14 which encodes NM II-C) are associated with human abnormalities. However point mutations in the gene encoding NM II-B (MYH10) have not been reported. We generated point mutant knock-in mice expressing motor-deficient NM II-B. Homozygous mice die at E14.5 in cardiac failure and exhibit novel abnormalities not seen in NM II-B ablated or NM II-B/II-C doubly ablated mice: a failure in midline fusion resulting in a cleft palate, ectopia cordis, and a large omphalocele, which resemble the human syndrome Pentalogy of Cantrell. Impaired apoptosis of mesenchymal cells in the fusing sternum and endocardial cushions contributes to these abnormalities. Expression of NM II-A and II-B, but not II-C in mesenchymal cells provides evidence that mutant II-B is interfering with wild type NM II-A function. One cardiac abnormality found in both mutant and NM II-B knockout mice relates to the cardiac outflow tract. The failure in myocyte disassociation exhibited by the II-B point mutants contributes to abnormal displacement of the aorta to the right ventricle in mutant mice resulting in both the aorta and pulmonary artery emanating from the same ventricle. The apparent cause is a failure in translocation of actin filaments by the mutant NM II-B motor that is required for disassembly of myocyte cell-cell adhesions. Specific expression of Rho kinase-1 in the fusing sternum and developing outflow tract cardiac myocytes suggests that this kinase mediates phosphorylation of NM II in these cells. Our studies show that activated NM II plays important roles in regulating apoptosis and disassembly of cell-cell adhesions during body wall closure and cardiac outflow tract alignment. They also emphasize the dual roles for NM II, one based on the crosslinking of actin-filaments and the other on translocation of actin-filaments. The failure in ventral wall closure resulting in cardiac externalization and the presence of a large omphalocele in our mutant mice has prompted us to study children with the diagnosis of Pentalogy of Cantrell for mutations in the genes encoding NM II-B and related proteins.